面试题22. 链表中倒数第k个节点

https://leetcode-cn.com/problems/lian-biao-zhong-dao-shu-di-kge-jie-dian-lcof/

func getKthFromEnd(_ head: ListNode?, _ k: Int) -> ListNode? {
    let dummy = ListNode(0)
    dummy.next = head
    var first = dummy
    var second = dummy
    var n = k
    //当前first停留在位置下标（k-1）上
    while n > 0 {
        first = first.next!
        n = n - 1
    }
    //继续遍历first，直至nil，此时second停留位置以后的节点即为倒数k个节点
    while first.next != nil {
        first = first.next!
        second = second.next!
    }
    return second.next
}